In recent years, it has been discovered that the impedance of a magnetic amorphous material changes due to an external magnetic field, and a magneto-impedance device (MI device) has been developed. For example, JP-A-7-181239 discloses a magneto-impedance device. A magnetic detection device utilizing such a magneto-impedance device has also been developed. For example, JP-A-2003-121517 and “Magnetic Sensor Technology” (Kaneo Mouri, Corona Publishing Co., Ltd., Mar. 10, 1998, pp. 92 to 101) disclose a magnetic detection device utilizing a magneto-impedance device.
An underground electromagnetic exploration method utilizing an electromagnetic induction phenomenon has been widely used for exploration of resources (e.g., mine, geothermal energy, and petroleum) and underground structure investigation. Various electromagnetic exploration methods have been developed. At present, an underground electromagnetic exploration method that artificially generates an electromagnetic field underground to implement underground exploration is used in practice. For example, JP-A-2002-71828 discloses an underground electromagnetic exploration method that determines the ground geological structure.
A time-domain electromagnetic method (TDEM method) is a representative underground electromagnetic exploration method. In the TDEM method, an induced current generating transmitter is disposed on the ground, and an alternating direct current having an ON/OFF time is caused to flow through the induced current generating transmitter. When the current that flows through the induced current generating transmitter is rapidly cut, an induced current flows through the earth's surface so as to prevent a change in magnetic field according to the law of electromagnetic induction.
The induced current diffuses toward deep underground with time. Since the induced current attenuates corresponding to the specific resistivity of the current path, the underground specific resistivity distribution can be determined by measuring the magnetic field formed by the induced current on the earth's surface as a function of time.